Negative ion source



Dec. l0, 1957 R. G. HERB ET AL 2,816,243

NEGATIVE 10N SOURCE:I

Filed April 9, 195e 4 sheets-sheet 1 llP /if :mili MIL III' Dec. 10, 1957 R. G. HERB ET Al.

NEGATlvE 10N SOURCE 4 Sheets-Sheet 2 Filed April 9, 1956 EXcf/A No; DEV/CE NEGAT/VE /oA/ BEAM Sau/20E o NEUTQAL A10/ws o/2 MoLEcUL/ss Pos/NVE /oA/s NEGATIVE /oA/s 012 ELECT/ZOALS Dec. l0, 1957 R. G. HERB ET AL 2,816,243

NEGATIVE ION SOURCE Filed April 9, 1956 4 Sheets-Sheet 3 l Illl Dec. 10, 1957 Filed April 9. 195e R. G. HERB ET AL NEGATIVE ION SOURCE 4 Sheets-Sheet 4 lresulting protons back to ground potential.

United States 19 Claims. (Cl. 313-63) This invention relates to a method of and apparatus 4for producing negative ions by the passage of positive ions through a gas. In particular, this'invention comprehends the production of negative ions by the passage of positive ions through a capillary into which a gas has been introduced at some point between the extremities of the capillary.

One of the most useful tools in nuclear research is an artificial source of high-energy charged particles, and one of the major problems in nuclear research is `the more ei'licient production of such charged particles. One method of producing high-energy charged particles makes use of the well-known phenomenon that negative ions can easily be converted to positive ions. As a result of this phenomenon, a high-voltage positively charged terminal can be used to attract negative ions, the negative ions `can be converted to positive ions within the highvoltage terminal, and the same terminal can be used 'to repel the resultant positive ions. In this way, the energy of the particles accelerated by an available voltage is increased; usually the energy of the particles is doubled. Moreover, both the ion source and the target are at ground potential. This method of producing high-.energy charged particles is disclosed in U. S. Patent No. 2,206,558 to Bennett, in U. S. Patent No. 2,213,140 to Kallmann, and in an article entitled Energy doubling in D. C. accelerators by Alvarez in the Review of Scientific Instruments, volume 22, page 705 (1951).

As a specific example of the aforementioned problem, the production of high-energy particles by electrostatic accelerators has become more difficult as higher accelerating voltages are sought. It is possible, according to .the aforementioned scheme, to double the energy of .protons from an electrostatic accelerator if one accelerates negative hydrogen ions to an electrode maintained at a voltage positive with respect to ground, strips' off the two electrons by passing the negative ions 'through a ,thin target inside of the electrode, and accelerates the In the same way, the energy of other positive ions from anv electrostatic accelerator may be increased.

An electrostatic accelerator of this type has additional virtues: maintenance should be easier than in present models since much of the machinery now housed in the high-voltage electrode could be kept at ground potential; the momentum of the negative ions can be determined such that only those ions that will ultimately be useful need be accelerated; there is also considerablefreedom of focal conditions before ,the beam enters the accelerating tube. Moreover, an electrostatic accelerator of this type may be used to attain pulsed proton beams of variable energies up to of the order of 100 m. e. v. or more,

v by means of an arrangement, to be described in more detail hereinafter, in which the negative ions are accelerated to high energy before being Vinjected into the electrostatic accelerator.

One of the A,main diflculties with thev executionvof this method is the provision of a negative ton source capable argenti() 2,816,243 Patented Dec. 10, 1957 ICC of yielding a beam of usable intensity for injection `into the' v'electrostatic accelerator or other. particle accelerator;

and, accordingly, it is an important .object of theA invention to provide a negative ion source suitable for` use in *the production ofhigh-energy charged particles by energydoubling methods. However, the invention is not limited to its use in such particle accelerators, `but includes, .all uses requiring a source of negative ions (e. g. spectroscopy). -An'arrangement whereby the negative ion source "ofjthe linvention may be used to increase the available v'energy for reactionsfproduced by charged particles arti ciallyaccelerated to energies in the billion-electron-volt energy such that the positive ions are converted into negative ions by the capture of electrons. Thin, foils or gases can serve as electron-donating `matter. yBecause only a few percent of the incidentpositive .ion beam is .converted to suitable negative ions, it is necessary to use an intense current of positive ions. The optimum positive ion energy for electron capture to Occur in foils is several kilovolts. The heat thus generatedv by such'abeam of positive ions would melt-any foil placed in the beam. The invention avoids this difficulty by Ausinga gas target as the electron-donating matter; in ,this

case the density of the donor material may bereadily adjusted to 4 l015 atoms per square centimeter, avvalue that approaches ythe density for charge exchange ,equilibrinm.

In the production .of negative ions by the passage ,of positive ions through a solid foil, the matter used for .the purpose may be only a few atom layers thick,` primarily because if the mass per unit area is great, the incoming positive ionslose appreciable energy, and energy straggling is thereby introduced. The use of a foil for .this

purpose is undersirable because it cannot be made thin v enough todo a good job. Gas, on the other hand, can

be made as thin as desired without fragility, and in this way one can minimize energy stragglingand scattering.

Moreover, since one usually Wants good conversion eciency and also a well-collimated negative-ion beam, it is necessary to use matter of low atomic number. ,In order to maximize conversion efficiency, ,the ionsl must emerge from the foil or other matter at low-energy, i. e. a few thousand electron volts. As a result, unlessthe matter is of very low atomic number, the positive; ions ,are scattered so as to be highly divergent in angle.1- With gas one can use low atomic number material suchtas hydrogen, `which gives an angular scatter one-sixteenth that given by beryllium.

Although considerations such as the foregoing lclearly indicate the desirability of using gas as the electrondonating matter, various diiculties have been encountered in attempts to provide an adequate numbenof atoms per unit area in the path of the beam of positive ions. Unless the number of atoms yper unit area in the path of -the beam of positive ions is at least a certain threshold value, a negligible number of negative ions will be produced. As the number of atoms perunit area in the path of the beam of positiveions is increasedabove the threshold value, the gain in negative ion yield vrises rapidlyand then approaches a saturation value where the negative ion yield rises at a diminishing rate. lIn ygeneral one, will operate in the vicinity of the saturation density.

`The invention overcomes the difliculties attendant, upon the achievement of thenecessary threshold valueby'di- 'recting the beam of positive ions through a capillary into which gas is introduced at some point between the extremlties of the capillary. The capillary provides more gas for a given flow, and increases impedance to gas ow,

l than, say, a container with entrance and exit aperture discs.

It is essential to the operation of the negative ion source, and is a big gain over previous devices which merely employed a chamber having two holes therein for positive ions should enter the capillary, and the acceleration of the positive ions should therefore take place in a space wherein the probability of the positive ions colliding with other particles is very small. Owing to the inefficiency of the conversion process, the ion current delivered to the capillary should be maximized; since the diameter of the capillary is very small, for reasons hereinafter to be set forth, the ion current density must be very high.

In accordance with the invention, these requirements are met by close spacing between the positive ion source and the capillary. The close spacing reduces space charge effects since it increases the voltage gradient for a given voltage dierence. The close spacing also reduces the probability for a given gas pressure in the positive-ion acceleration space, of the positive ions colliding with other particles before reaching the capillary. The spacing, however, must not be so close as substantially to impede the iiow of gas from the capillary into the exterior evacuated region.

A further important feature of the invention is the way in which negative ions are extracted from the electrondonating matter. The device which accelerates negative ions from the capillary will also accelerate electrons. The acceleration of these electrons absorbs power from the negative-ion-accelerating device, and the accelerated electrons produce X-rays when they strike an object in their path. In accordance with the invention, an electron suppressor is provided to stop these electrons before they have acquired appreciable velocity; in this way, power absorption and X-rays are minimized. The ex pense of operation would be prohibitive without using the electron suppressor of the invention.

These and other important features of the invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which:

Figure l is a somewhat diagrammatic view of longitudinal central section of a negative ion source embodying the invention;

Figure 2 is a diagram illustrating the main parts of a negative ion source embodying the invention;

Figure 3 is a view in longitudinal central section of a modification of a portion of the negative ion source shown in Figure l;

Figure 4 is a diagram illustrating apparatus in which negative ions are iirst accelerated to' high energy by a linear accelerator and then injected into an electrostatic accelerator lin which they are either accelerated or decelerated and converted to positive ions, which are then further accelerated or decelerated;

Figure 5 is a diagram illustrating a protron synchroton in which positive ions and negative ions are accelerated in opposite directions and allowed to collide at the same energy in the b. e. v. range; and

Figure 6 is a diagram similar to that of Figure 5 and showing a modification of the proton synchroton of Figure 5.

Referring to the drawings, and rst to Figure 1 thereof, positive ions are produced yby an ion source 1. These ions are extracted and accelerated by a voltage of the P order of 104 volts. The ions then pass through an cleo 4 tron pickup capillary tube 2 into which the electron do hating gas is introduced. The negative ions emerging from this tube 2 are further accelerated, while the emerging secondary electrons are electrostatically repelled by an electron suppressor 3. The negative ions are then focused by a suitable lens 4, shown in Figure 1 as a saddle-field lens. The entire negative ion source, excluding the vacuum pump and magnetic analyzer assembly, is shown in Figure l, and is enclosed within a chamber 5 which is evacuated by the vacuum pump.

The negative ion source may be constructed entirely of metal and ceramic so that it can be operated at the elevated temperatures associated with high power input. The entire negative ion source assembly, excluding the focus lens 4, is fastened to a demountable end iiange 6, so that this assembly can readily be removed for servicing. Metal gaskets may be used on all demountable joints. The positive ion source 1 is constructed so that the plasma-enclosing envelope is entirely housed in the vacuum chamber 5, thus minimizing the number of vacuum seals which are exposed to atmospheric pressure.

The positive ion source 1 may be any one of the various well-known types. Merely by way of example, there is shown at 1 in Figure l a magnetic ion source, similar in principle to the one described by I. Kistemaker and H. L. Douwes Dekker in Pity/Sica, XVI, 3, 198 (1950). A suitable gas, such as hydrogen or deuterium, is introduced into the positive-ion source through a suitable gas line 7. Electrons are emitted by a spiral filament 8 and drawn toward a cylindrical anode 9, which is maintained at a potential of between 0 and 300 volts positive with respect to the filament by a voltage source 10. However, an axial magnetic iield produced by a solenoid 11 constrains their motion along the axis. The electrons thus move in the axial direction until they reach the end of the ion source 1 where they are reflected by a plate 12 held at filament potential. This arrangement is very eflicient for producing a dense plasma. The location of the filament 8 near the positive ion exit aperture 13 modifies the plasma boundary so that a large yield of positive ions is assured for a given extraction voltage.

The positive ions are withdrawn from the plasma through the aperture 13, and focused into the capillary tube 2 by means of a voltage source 14 which maintains the electrode 15 in which the capillary 2 is formed at about l()4 volts negative with respect to the iilament 8. To reduce space charge current limitation it is desirable to maintain the positive ion extraction gap as small as possible. The distance separating the ion source exit aperture 13 from the capillary 2 should be between l and 10 times the diameter of the capillary Z, and preferably is 2 or 3 times the diameter of the capillary 2. The half of the electrode 15 that is bombarded by positive ions may be constructed of molybdenum because of its high melting point and machinability, while the other half may be of aluminum. Instead of molybdenum, berryllium copper may be used, since it has similar properties, is cheaper, has a high thermal conductivity, and provides a high yield of secondary electrons to help neutralize space charge. A suitable electromdonating gas, preferably hydrogen although other gases, such as argon, may be used, is introduced into the pickup capillary 2 between the extremities thereof through a suitable gas line 16. Preferably the gas is introduced at the center of the capillary 2, as shown in Figure l; and, where the electrode 15 comprises two parts, as just described, the gas may be introduced be tween these two parts, as shown in Figure 3.

The beam that ieaves the electron pickup capillary tube 2 is composecfl of negative ions, neutral particles, positive ions and electrons. An electron suppressor 3, which may comprise a disc with a 3/8 inch diameter aperture held at 50 volts negative with respect to the electrode 15 by a voltage source 3.7, is located a relatively large distance, such as 3A inch, from the exit aperture of the capillary 2. This distance is chosen large so that the secoudasi-imacat;

ary electronsiproduced.v in the. gasflowingrfrom'the pickup"- tubelZare also. suppressed. The'aperture inthe electron 't suppressor 3 is chosen large iu comparison'tothe. diame-i terfofi the beam-:so: assto minimizeV spherical' aberration atzthe entrancerrtowthe negative ion accelerating gap.'

Itkgisfzdesirable `tof accelerate #the negative ions to `as great.v an ienergyfas:possiblebefore they enter .the electro-v static accelerator or other particle accelerator since theyV thenawill:bea'deected-less by spurious charges thatsmay buildtupxoni the, -insulators nearv the entranceof 'the accelerating :tubes: Abbes sine law'also indicates that the higher .the rbearrrfenergythey smaller. will be '.theLbeam di-z ameter, and the. moreparallelit will beto-the axisof the. acceleratingftuber- For zthese.l reasons.vv the positiveffion sourcer.1, electrode'lvand electron suppressor 3 may be biasedebetween` Oz'and 45 kilovolts'negativewith respect toground1byrthe voltagesource.18.:` :This assembly may` be insulated by an 8 inch O. D. by 6.inchI..D.. byA 2- inch ceramiciring 19,Jand'the.associatedtpower supplies 10,114, l'lf-mayfbetisolated from ground -throughstransformers (notishownywith 45"l;ilovolt insulation.-

The negative ion accelerating electrodewZtlmay. consist.A

of'gafl/z'inch diameter cylinder," 8 incheslong,.which isi" atqgroundpotentials This cylinder, yalong-withfthe--aperf tureofigthefelect-ron 4suppressor 3, define; the negative ion accelerating region. The `diameterfis chosenfhlaragecto` minimize: sphericalaberration,v and to.k spreadthe'accelerat-.

Vi=energy of the beam .within the electrode f Vov=energylof the beam outside'of--the electrode, i. e. the

nalfenergy of the beam Thiselectrode4 may have, for example, a 4 inchdiame.- ter entrance .and a 1% inch diameter.exit....A suitable-. length might be ofthe .order of ..7 inches, since..a..longer. electrode4 would Anot give .a vshorter .focal length...for.:a. given applied voltage... All ofthe electrodesS, 3,20-,

and.4 are. reentrant, so that thebeam is .shielded.fro`m.

any.,.electrostatic potential. asymmetries. The beam may` nowpassthrough a 2%, inch diameter. stainless steelitubev 2.12.,.and may then beA deected by a beam analyzer assem.. b y.

summarizing, the production oflnegative-ions by causing positive ions to travell through matter involves the use of apparatus which may conveniently be divided into fourjmainparts'as follows: (l) the source of positive ions;.(2), the exchange device; (3) means to deliverv thev positive Vi'ons .to the exchange device; and (4) meanstodeliver f thenegative ions from the exchange device. Suchi a Anegative .ion .sourceis illustrated in the `diagram' of Figure..2. Referring thereto, positive ions .produced in anion source..1 are deliveredinto an exchange device 2` by-means ofa suitable accelerating mechanism-13;.. Negaf., tive ions which are produced. in the. exchange device. 2. are delivered therefrom bya suitableaccelerating mecha. nisrn-,.24..Inl general, the emergent :negative-ion beam willbe composed vof more than one type of negative ion; and:so1-the 'type of negative ion desired may be separated:v from the rest ofthe beam .bydirecting thebeam intera*l beam analyzer 25,; whence A.the .desired `negative-ions emerge as a beam `26....

Ther principalvrequi-rement :imposed on the positive y ion. 7.0

soureeinthe ,case of vapparatusconstructed in accordancewithithe. invention; is. that it should. have good luminosity, i where? luminosity fis fa quantity/which. is proportional@ to the .tionfcurrenttperunit area...v The luminosityiis more?.

current "is-:that .whichr-lcan abe delivered through the* capillaryf While 'the Iinvention is not limited to any particular type of positive ions or to any particular gas source for? thepositive ions,-the .invention will often-be used in connection with vthe production of positive ions from some V isotope of hydrogen.. The ion beam vfrom a positive ion source utilizing hydrogen'vof atomic weight 1 is composed of three-:typesof-molecular ions, `being mass 1, mass 2\ and massS. c Among' these, the order of preference may@ be: pure` mass 3, puremass 2, pure mass 1, mixed. cIn. practicegrone' maya'aimfor mass 2.` The-higherlweight-= is desired because it gives-two bombarding particles, whicl1- tendsto double uthe:yieldf of-negative lions vwhile the-accompanyingreduotiondue to vincreased space chargef ef. fects lonlyl/Z'. lSincefthe'energy is lshared between tw'o'I particles; 1 the. accelerating lvoltage -must be doubled. This f is easily.'zachieve'df,up-. to,` :values where electricall break-i down= occurs between thefcapillaryand the positive ion source; because ofvthexrelatively low voltagesinvolved.y

If,deuterium :gas is :used -in the positive ion source, the massoffeach typerofrion is'twice vas great as'when hydro-- gentgas isused;y and-theyield of negative deuterium ions' at::anrextractor:.voltage Vco'rresponds rather closely to the'.yieldf'of.hydrogen-ions at'avoltage of-'V/Z. To gethigh yieldsfofi'deuteriumionsgit is necessary to go-l tof highe'renergiesthat is; to higher'voltages on the extractorx elect-rode 15'.l This .is-tube expected because for efficient capture of electrons the @ions must befmovin'g at a velocity. corresponding'.approximately to the velocity of the outer-` mostielectrons inthe atoms'of the'converting gas.

In onetembodime'ntwof: .the invention, lthe positive-ionsoureeris :anfimprovement of the Kistemaker source,-fas hereinbefore described. In this source the ions produced aren mostly notfmonatomi and, l"as noted'` aboveyfthat VVis desirable. Because' -of the highnluminosity needed,` the. ion/source-.will be'delivering about-500 lwatts on a smalle spotgfand.v so lhighftemperature -construction:must:v be used.` i

Vaporswould cause-fthe filament' to` burn'fout rapidly-,. and s0 the=ion source .shown'in Figure lmust beivaporf. free; 'in orderthat it maybe taken up to :high temperature without. damagef.. ',In accordance with the "invent-ion,fthe ion source'.Y is fall. metal and ceramic, i. andy henceV dry, because.it is Afree of all materials, such as-.organic materials; with highvapor pressure. Onlyi-low-vaporfpressuremate-g rials,-. like metal e (e.y g; molybdenum, stainless steel; alumi-.f numyand-ceramic (eng. aluminum-oxide), are used.

WhileA two gas sources'are preferred, it .is possible tov use-at-sngle'gas vsource-.for both the positive` ion source-- and the'fcapil-laryr If one gassource is-used,.-the.drift.v shouldpreferably be. from thecapillary torthe iontsourcef although the `reverse is possible.-f This is because, as notedV above,f thecapillarywpressure should' be highbuttho-` space between the capillary and the positive yion source must 'beaevacuateda Fortunately, the maximumpermissible ypressure in the". accelerating space'exceeds theminif.- mumrperrnissible'rpressure for the .positive ion.` source.

The.number of:neg`ative ions :produced by the passage.. of..awgiven.beam-.ofnpositive ions .through a .given..ex1.. change.; device isara function of-the--energy of.. thezincoming'positive-fionsf-the number of ynegative ions reachesV a maximum "at-.a ypositive-.ions .energyof a few thousand'. electron volts;r .Of .--course,` the energy of. the .negative ions.. is also-.a\ functionofrthe energy-vof the.incoming.positive ions,beingapproximatelyequalthereto.v Hence the posi. tive ions ln1ust.fbe acceleratedasthey Yare delivered from the? positive a ion source .to .the exchange device, .and .the`.. positivedon zbeam should besubstantially monoenergeticr.v This means that thespaceabetween thepositiveion source and vthelexchangeadevice mustbe evacuated.

Moreover; mostY practical .applicationsA kof the negativet. ion source-will.requirefthat thespace into which the. negativelr=ions...emerge from' the. exchange? device also be r evacuatedzfw:A 1

importantzthan .the; intensity, becausezztherlonly useful ionea7 In orden toLf-maintainftheproperxdegree ofavacuun'einf`A order to remove from these spaces the gas which constantly flows' into them yfrom the capillary 2 and from the ion source i.

At the pressure to which these spaces must be evacuated, the maximum pumping speed (liters per second) available limits the throughput or gas flow (pressure times volume per second) tolerable. As hereinbefore noted, a certain minimum or threshold number of atoms per unit area must be presented to the positive ion beam. in thev capillary 2. The number of atoms per unit area is proportional to the integral, over the length of the capillary, of the pressure within it. Therefore, since the pressure at the extremities of the capillary is limited to a certain maximum value by evacuation requirements, the number' of atoms per unit area can be increased for a given capillary only by increasing the pressure at the point where the gas enters the capillary. However, although this pressure may easily be increased, such increase will also increase the gas iiow through capillary by a factor which is proportional to the conductance of the capillary. It there fore follows that the permissible gas ow is increased by decreasing the conductance of the capillary. Since the conductance of the capillary is proportional to the ratio of the cube of the diameter of the capillary to the length thereof, the capillary diameter should be decreased as much as possible and the length increased.

However, limitations on these dimensions are imposed by other considerations. We believe that space charge effects within the capillary may be reduced by trapped electrons. There is much ionization and secondary electrons are also knocked from the walls. We believe this source of negative charge may prevent the potential inside the capillary from departing appreciably from the potential of the wall. Since the conversion of positive ions to negative ions is relatively ineflicient, it is important to maximize the current through the capiliary. How many ions get through the capillary will depend on how many enter the capillary and on how nearly parallel their paths are to the axis of the capillary. Some divergence is to be expected. The best ratio of the radius r of the capillary to the length L of the capillary will be determined by (a) the angular divergence of the beam and (b) the luminosity at the entrance to the capillary as a function of distance from the central axis of the capillary. Combining the limitation on the ratio r/L with that imposed by the necessity for impeding gas flow, it is seen that the diameter of the capillary must be made very small, and that the length cannot be made too long. Where space charge effects are eliminated, the maximum permissible ion current which can pass through two apertures of radius r separated by a distance L is the same, whether the apertures are joined by a large chamber or by a tube of radius r and length L.

As a result of these limitations, the pumping speed will have to be high. The high pumping speed is an important feature of the invention, and is required because of the upper'lirnit on the length of the capillary imposed by space charge considerations. In general, the pumping speed will have to be at least of the order of 10 atmosphere-cc. per hour, or at a pressure of the order of105 mm. Hg, the pumping speed should be at least of the order of 1()3 liters per second. In some embodiments of the invention, for example, when the positive ion source is of a type having a heated filament, vapor-free conditions are desirable; and, in such cases, it may be advantageous to use a high-speed dry pump, such as that described in the Review of Scientific Instruments, of volume 25, page 1193 (1954) by R. H. Davis and A. S. Divatia.

It has been noted that the gas pressure at the inlet to the capillary 2 should be made as high as is consistent with the gas ow resulting. In this respect, the capillary has marked advantage over a larger chamber into which and from which the ion beams pass through apertures, because theinlet pressure may be raised much higher for a" given outlet pressure in the case' of a capillary than iny the case of an enlarged chamber. This, in turn, increases the massper unit area of gas. f

' Moreover, as noted above, where there are no space.

charge effects the maximum permissible current through a tube is the same as that through any larger chamber having inlet and outlet apertures of area equal to the tube cross section.

Space charge effects are also reduced by secondary electrons which are emitted from the metal surface of the conversion tube, and which neutralize the space charge of the positive ions. At low velocity, electrons are good at neutralizing space charge. Secondary electrons which are emitted from the inner wall of the capillary are shielded from all electric fields except that due to the positive ion beam, and so their velocity is particularly slow and hence they are particularly effective in counteracting the eiects of space charge.

Since, as hereinbefore pointed out, the positive ion source 1 produces positive ions of various masses, the negative ion beam which is extracted from the capillary 2 will also include negative ions of various momenta. The magnetic analyzer 25, shown in Figure 2, eliminates from the beam 26 all but negative ions having the momentum desired. Thereafter, the negative ion beam 26 can be injected into a suitable acceleration tube, or any other device for which the negative ions are desired. Of course, the analyzer 25 is not an essential part of the invention.

It should be noted that the negative ion source of the invention provides a negative ion beam of small divergence.

As previously noted, the lens 4 shown in Figure l may be any type of lens, but by way of example is shown as a saddle-eld lens. Such a lens is preferred on account of its simplicity.

In the construction shown in Figure 1, the electron suppressor 3 has a relatively small aperture and is biased at about 50 volts negative with respect to the electrode 15, while the negative ion accelerating electrode 20 presents a relatively large aperture to the entering ion beam. These three electrodes, 15, 3 and 20, act as a lens which has a cross-over; and there is some danger of an objectionable space-charge spreading of the beam at cross-over. To minimize this danger, the modied construction shown in Figure 3 may be employed, wherein the electron suppressor 3 is biased at about 100 volts negative with respect to the electrode 15, and wherein the negative ion accelerating electrode 20 presents a relatively small aperture to the entering ion beam. Such a construction reduces the diiculties attendant upon cross-over.

The electrode 15 having the capillary 2 in which charge exchange takes place gets heavily bombarded by positive ions, and its temperature rises. For a given number of atoms or molecules in the capillary 2, the number that ilows out per second will increase as the gas temperature increases. Cooling the converting gas therefore has certain advantages. the input gas to the conversion tube 2 or by cooling the electrode 15 having the conversion capillary 2. This latter procedure would probably be the more practical of the two possibilities. in either event, the cooling may be accomplished by techniques well-known in the art, and need not be described herein in any detail.

The diagram of Figure 4 illustrates a scheme for attaining pulsed'proton beams of variable energies up to the order of m. e. v. or more. Referring to said Figure 4, a negative ion source 27, which may be constructed in accordance with the invention, produces a negative ion beam which may be injected either into a linear accelerator 28 or into a double-ended electrostatic accelerator 29, depending upon which the deecting magnet 30 is turned oli or on, respectively. The electrostatic accelerator 29 has a high-voltage terminal 31 which is maintained at a positive potential and which contains a suitable device 32 for converting negative ions passing there.

This would be accomplished by cooling through into positive lions, so as.. torprovide energy.A

doubling .as hereinbefore described. Assuming that the negative ion source 27l is a :hydrogen source,- and ifvthe potential of the high-voltageterminal 31 can-be varied`l between and 5 megavolts, the electrostatic acceleratorV 29 will furnish 'a continuously variable beam l,energy fronr Oto l0 m. e. v. when the magnet -30lis energized so as to deflect negative ions from the negative'ion source-27` into theelectrostatic accelerator 29.

When the magnet 30 is turned oli?, the negative ions: from the negative ion source 27 are directed into a linear accelerator 28 which may be .fof-the same design-as con-` ventional proton linear` accelerators except for a suitablealongv to serve as the second electrostatic.accelerator 34.

Since the polarity of the belt-charging system of an elec- `trostatic generator can be easily reversed, the potential of the high-voltage terminal 36 of the second electrostaticv accelerator 34 can be varied between -5 and +5 mega. volts. If the rst linear accelerator 28-has a 205m. e. v.- output, the second electrostatic accelerator 34 will furnish a continuously variable beam energy from to 30 m. e. v. when the second magnet 35 isenergized 2solas to deect negative ions from the trst linear accelerator 28 into the second electrostatic accelerator 34. Of course7 when the high-voltage terminal 36 of the second electrostatic accelerator 34 is at a negative potential, theiofnsl traveling through the second electrostatic accelerator 34 are continuously decelerated rather than accelerated.

By inserting another -m. e. v. linear acceleratorsection 33 and following this with a double-ended electror static accelerator `37, the beam energy ymay be made toy vary from to 50 m. e. v., and so on. In this manner one may get a proton beam continuously variable from .0.

to Whatever energy one desired within the limitationspf the linear accelerator. Such avariable energy-control would overcome the most serious drawback ofnpresent' mate energy from an electrostatic machine from the high.

voltage insulation problems.

Of course, the arrangement just described is not limited to the production of proton beams, but includes the production of beams of other positive ions as well.

In doing b. e. V. proton experiments apparently one limitation on increasing the available energy for reactions is the fact that more energy goes into center of mass motion (i. e., the relativistic mass increase effect). It has been proposed that this difficulty could be circumvented by allowing two beams to collide with b. e. V. energies.- i This could be done by building two very costly machines and allowing their beams to collide head on.

The diagram of Figure 5 illustrates an arrangement whereby the considerable expense of a second b. e. v. machine may be avoided in accordance with our invention. Referring thereto, both positive and negative ions are introduced into a single proton synchrotron 3S in opposite directions from the positive ion source 39 and a negative ion source 40, respectively. Both ion beams are accelerated in the same machine 38 and are allowed to collide at the same energy in the b. e. v. range, the collision taking place at some convenient location such as 41.

In the diagram of Figure 5 the positive i-ons and negative ions follow different paths during a substantial portion of the acceleration period, and they are then redirected so that some collide in the collision region 4l. In the diagram of Figure 6 the positive and negative ions follow the same path but in opposite directions. Assumion lsolulceffztfand theinegativedornsource twiareibunched,

as indicated; at- AZxand, 431;respectivelyy southatficollisions: are conned to particulariregionsflfl where. `collisionevents; can Ibef, investigated. i 1:;l

The ion beamsgare .not appreeiahly alternatcdfgbynthei relatively rare -nuclear collisiousn v Jn any, imach-ineghigh; energy ions will beglos-t `m08tlyaby factors;suchvftasagas; scattering-and escape. fromxfthe; orbita; The ,practicabilityzz offthe;machineiWOuldggbe determinedxmostiy' byfihe: PIO abilityghof' negative. .ionl0ss;due\.to encountersiwthior iv; nary 'gas atomsQingthe.,A acceleratore/avi AsY a vrresulhva `,com-v` paratively. low pressuregas VSM10-r1mrn. of Hg',;isire,-;: quired in the accelerator.

It hasfbeenstated;hereinbeforethat :a veryffast pumping Speed is:fordirrarilyeirequired i connectifemwth ithef negar;

f tive i011- sour ffoffthezniten, enmzHowever, ymanyyapplig cations of thenegatiy iionfslourceiwill contemplate pulsed-,f operation.: v Where -thisyisthegcasapumpingopeed req1.1ir`e:v ments may-be relaxed,;;in;;acc,ordan cewith thetinyenfionn byy using-a pulsed positiveionfsource l with, inlsomegcases.. a pulsed gas source for theiexchangef.-device v2' ;f'(sec,: Figure q2) y.

Having thus fdescribedffthe; principles; of the invention, i together` with.v 1 trative '.embodiments, thereof, .it f is :togA be: understoodLthatilalthough:speeicsterms. are employed-,e

" th,ey .areused,^,i a generic-aridil descriptive; sense and, not..

forfpurposes o f-lin-ritatiota-the,fscopeqof; the Iinvention being set forthin theftollowingclaims.-` f.

We claim:

1. j A-;negative tienlsourceicomprisugtnicombination:i a

chambrgacfpump gadapted; to evacuatesaid;v chamberyank electrodel supportedffwithn :Said .ichamberf-.and.v havingxa. capillary thereth r o1,1gh,l rrieausrfor,y introducing-a gasainto; Said, apillaryiatsome point: betweenthe, .extremities therev of, means y fon-creating@ beamfo` positive*lions,-y and means; for directinggsaid gbeama-,intoefsaid-rgcapillaryi lengthwiset` thereeip 2. A negative ion source in accordancemyvith;plaintiff-L; Whercimsaid purnnhasta;pumpingispeedtofimere thantlO3 litersfpensecondrf i V3. A; -negatiyeionsourcein; accordance `with-',clairn il, wherein. said :gas comprises arnisntope `of-.1hydr.ogen andi wherein saidl beam1=comprises positive; :ions'havingaa ysingle nuclearcharge.;A v

4. Av negativepionwsource.comprising -`infcornbination;; a chamber, Ja .pump adapted ato evacuate tsaid ',chambrn anV electrode, supported@ :within: said chamber:land;y having: a capillary.therethrougmmeans `for introducing a gaspinw; said capillary.atsomepoinnbetweenftheiextremitiesatherea of, a positive ion sourceincludingan,enclosurehavinganr aperture`- therein-aand. means fforfproviding ipositivexion-s in the vicinityif said aperture-ifsaideenclosureebeingifposif; tioned-gtso, r that Said: aperture :is iclosezato: one endf ofesad; capillary,I andmeansgfor maintainingzsaid;electredefat a potential iofnthe oriderooff, voltsfn'egatiyez V.withf:respecta to said enclosure.

5. A negative ion source in accordance with claim 4, wherein the spacing between said aperture and the nearer end of said capillary is between one and ten times the diameter of said capillary.

6. A negative ion source in accordance with claim 4, wherein the spacing between said aperture and the nearer end of said capillary is approximately three times the diameter of said capillary.

7. A negative ion source comprising in combination: a chamber, a pump adapted to evacuate said chamber, an electrode supported within said chamber and having a capillary therethrough, means for introducing a gas into said capillary at some point between the extremities thereof, means for creating a beam of positive ions, means for directing said beam into said capillary lengthwise thereof,

means for extracting negative ions from said capillary, and means for collecting those electrons which are accelerated by said means for extracting negative ions, prior to their acquisition of appreciable energy.

8. A negative ion source in accordance with claim 7, wherein said means for extracting negative ions comprises an extractor electrode which is maintained at a potential of the order of 104 volts positive with respect to the electrode having said capillary, and wherein said means for collecting electrons comprises a suppressor electrode between said extractor electrode and they electrode having said capillary, which is maintained at a potential of the order of 102 volts negative with respect to the electrode having said capillary, said extractor electrode having a relatively small aperture, whereby space-charge effects of cross-over of the ion beam between said capillary and said extractor electrode are minimized.

9. A negative ion source comprising in combination: a chamber, a pump adapted to evacuate said chamber, an electrode supported within said chamber and having a capillary therethrough, means for introducing a gas into said capillary at a point approximately midway between the extremities thereof, means for creating a beam of positive ions, and means for directing said beam into said capillary lengthwise thereof.

10. A negative ion source comprising in combination: a chamber, a pump adapted to evacuate said chamber, an electrode supported within said chamber and having a capillary therethrough, means for introducing a gas into said capillary at some point between the extremities thereof, means for cooling said gas, means for creating a beam of positive ions, and means for directing said beam into said capillary lengthwise thereof.

11. A negative ion source comprising in combination: a chamber, a pump adapted to evacuate said chamber, an electrode supported within said chamber and having a capillary therethrough, means for introducing a gas into said capillary at some point between the extremities thereof, means for cool-ing said electrode so as to cool the gas in said capillary, means for creating a beam of positive ions, and means for directing said beam into said capillary lengthwise thereof.

12. In combination with an ion accelerator having a fixed-energy output which cannot be altered by the operator of the ion accelerator: means for injecting negative ions into said ion accelerator so as to be accelerated by the same, conversion means for converting negative ions yto positive ions, means for maintaining said conversion means at a fixed potential which can be varied by the operator of the ion accelerator, and means for directing the high-energy negative ions issuing from said ion ac-V celerator through said conversion means, whereby positive ions are produced having an energy which can be varied by the operator of the ion accelerator.

13. In combination with an ion accelerator adapted to accelerate ions by means of at least one high-frequency electric iield impressed upon a portion of the trajectory of said ions, said portion being xed in space and said tield being synchronized with the velocity of said ions, so

that said ion accelerator has a fixed-energy output which cannot be altered by the operator of the ion accelerator: means for injecting negative ions into said ion accelerator so as to be accelerated by the same, an electrostatic belttype generator having an electrode upon which is maintained an accumulation of electric charge the amount of which can be varied by the operator of the ion accelerator, thereby varying the fixed potential of said electrode, conversion means, supported at said electrode, for converting negative ions to positive ions, and means for directing the high-energy negative ions issuing from said ion accelerator through said conversion means, whereby positive ions are produced having an energy which can be varied by the operator of the ion accelerator.

14. Apparatus in accordance with claim 13, wherein said ion accelerator comprises a linear accelerator.

15. Apparatus in accordance with claim 13, wherein said ion accelerator comprises a plurality of linear accelerators adapted to act cumulatively upon said negative lons.

16. `ln combination with an ion accelerator for accelerating ions to energies at least of the order of 109 electron volts: means for injecting positive ions into said ion accelerator so as to be accelerated by the same, means for injecting negative ions into said ion accelerator so as to be accelerated by the same, and means for causing at least a substantial portion of said positive ions to collide with said negative ions after said positive ions and said negative ions have each acquired energies at least of the orderof 109 electron volts.

17. Apparatus in accordance with claim 16, wherein said positive ions and said negative ions follow different paths during a substantial portion of the acceleration period.

18. Apparatus in accordance with claim 16, wherein said positive ions and said negative ions follow substantially the same path but in opposite directions, and wherein means is provided to confine the ion beams to a narrower land narrower pencil as ion energy is increased, so that collisions will be infrequent until the ion energy becomes high.

19. Apparatus in accordance with claim 16, wherein said positive ions and said negative ions follow substantially the same path but in opposite directions, and wherein means is provided for bunching said ion beams, so that collisions are confined to particular regions where collision events can be investigated.

References Cited in the tile of this patent UNITED STATES PATENTS .2,232,030 Kallmann Feb. 18, 1941 2,489,344 Washburn Nov. 29, 1949 2,498,841 King Feb. 28, 1950 2,570,124 Hernquist Oct. 2, 1951 2,642,535 Schroeder June 16, 1953 2,643,341 Leland June 23, 1953 2,677,061 Wilson Apr. 27, 1954 

